1. Technical Field
The present disclosure relates to a constant on-time converter, in particular, to a constant on-time converter having fast transient response.
2. Description of Related Art
For the power management in a system, constant on-time converters are often used to provide different levels of operating voltage. A better constant on-time converter is capable of providing a stable output voltage and a wide range output current. When the load changes instantaneously, the output voltage can still be stabilized at the original voltage level and quickly provides the corresponding load current, thereby efficiently converting the voltage.
Reference is made to FIG. 1 which shows the diagram of a traditional constant on-time converter. The constant on-time converter 10 is used for converting the input voltage VIN into the output voltage VOUT to drive a load (representing by the capacitor Cp). The constant on-time converter 10 includes a switch circuit 12, a feedback circuit 14, a pulse width modulation (PWM) controller 16, and a driving circuit 18. The switch circuit 12 includes a high-side switch Sup and a low-side switch Sdn. The high-side switch Sup which is turned on provides a charge path to an inductor L. The low-side switch Sdn which is turned on provides a discharge path to the inductor L. The feedback circuit 14 is used for detecting the variation of the output voltage VOUT. More specifically, the feedback circuit 14 divides the output voltage VOUT by two series resistors A1 and A2 to generate the corresponding feedback voltage VFB to a decision circuit 16A of the PWM controller 16.
The decision circuit 16A generates a switch signal SW to a clock generator 16B of the PWM controller 16. The clock generator 16B generates the clock signal CLK with the constant on-time to the driving circuit 18 according to the switch signal SW. The driving circuit 18 controls the high-side switch Sup and the low-side switch Sdn according to the clock signal CLK to charge or discharge the inductor L. Therefore, the driving circuit 18 generates the necessary load current and the stable output voltage VOUT.
However, in the architecture of the traditional constant on-time converter 10, the clock signal CLK with the constant on-time is a periodic oscillation signal. It usually needs to have a minimum off-time for over current protection. Therefore, when the load is instantaneously converted from light load to heavy load (e.g., the load current is converted from 1A to 6A), the clock generator 16 generates the clock signal CLK having the minimum period, causing an excessive decrease in the output voltage VOUT to reduce the transient response of the output voltage VOUT.
Reference is made to FIG. 2 which shows the diagram of the load of the current mode voltage converter being converted from light load to heavy load. As shown in FIG. 2, at time point T1, the load is converted from the light load to the heavy load. At this time, the feedback circuit 14 detects the variation of the output voltage VOUT, and the clock generator 16B generates the clock signal CLK having the minimum period to rapidly increase the current flowing through the inductor L. During time points T1-T2, the output voltage VOUT has an excessive decrease because the clock signal CLK needs to have the minimum off-time. After that, the driving circuit 18 gradually stabilizes the output voltage VOUT to a voltage level in the subsequent clock signals (e.g., the clock signals of the time point T3).
Similarly, reference is made to FIG. 3 which shows the diagram of the load of the current mode voltage converter being converted from heavy load to light load. When the load is instantaneously converted from heavy load to light load (e.g., the load current is converted from 6A to 1A), the clock generator 16 generates the longer off-time to rapidly reduce the output voltage VOUT. More specifically, as shown in FIG. 3, at time point T4, the load is converted from the heavy load to the light load. At this time, the feedback circuit 14 detects the variation of the output voltage VOUT. The clock generator 16 generates the longer off-time to rapidly reduce the current flowing through the inductor L. The driving circuit 18 gradually stabilizes the output voltage VOUT to a voltage level in the subsequent clock signals (e.g., the clock signals of the time point T5). Therefore, in the architecture of the conventional constant on-time converter 10, the transient response of the output voltage VOUT does not reduce.
As mentioned above, when the load is instantaneously converted from light load to heavy load, the output voltage VOUT of the conventional constant on-time has the worse transient response.